This application relates to systems and methods for converting a liquid into a vapor.
Fuel cells, fuel processors (e.g. reformers, burners and catalysts) and gas purification systems utilize various gaseous reactants. Depending upon the system configuration, gases such as hydrogen, oxygen, carbon dioxide, nitrogen and methane may be employed in different proportions. For many of these configurations it is critical that gaseous influents supplied to the system be accompanied by a minimum amount, of humidity. Failure to maintain an acceptable water to carbon ratio can lead to carbonization of the system and degradation of fuel cell or fuel processor performance, particularly in the case of molten carbonate and solid oxide fuel cells, and gasoline, diesel and natural gas reformers, where gases are delivered at very high temperatures. For example, without appropriate amounts of humidity, fuel cell membranes in proton exchange membrane (PEM) fuel cells can lose their property to transport protons and might eventually become brittle and break.
Conventional vaporizer designs include a reservoir of liquid that is vaporized and replenished. However, the amount of liquid in the reservoir at any given moment is not accurately measurable. Vapor metering is possible but it is complex and impractical for many applications. Typically expensive mass flow meters and control valves are required. These instruments have limits regarding operational ranges of pressure, temperature and flow turndown. Moreover, injecting or spraying liquid on to a hot surface is not workable for large liquid flows due to the large amount of heat and surface required to vaporize all of the liquid (and avoid liquid pooling). Liquid pooling is undesirable because the collected liquid does not participate in the vaporizing process and thus has to be purged at intervals.
The need has therefore arisen for a system and method for converting a liquid into a vapor that avoids liquid pooling, and that can accurately meter the amount of liquid converted into vapor.
Described herein is a system for converting a metered amount of liquid into a vapor. The system includes a housing, a liquid receptor, that is preferably porous, disposed within the housing for accepting the liquid, a liquid inlet for delivering the liquid from outside the housing to the liquid receptor, and a measuring device for determining the amount of liquid delivered to the liquid receptor. The system further includes a heater for heating the liquid delivered to the liquid receptor to thereby convert the liquid to a vapor, and an outlet for discharging the vapor outside the housing.
The housing defines an internal chamber, in which resides the liquid receptor. The liquid receptor can be composed of a porous ceramic. The liquid receptor has a sufficiently large wettable surface area and temperature to evaporate all of the liquid introduced onto the liquid receptor, thereby avoiding liquid pooling. Liquid travels from the introduction area(s) of the liquid receptor to more remote areas of the liquid receptor by gravity feed and capillary forces.
In one embodiment, the liquid receptor may have the shape of an elongate hollow cylinder. The liquid receptor may be heated in many different possible ways, such as using radiant, induction or convective heating. For example, a cartridge heater may be positioned within an internal cavity of the liquid receptor. Additionally or alternatively a plurality of band heaters may be mounted on the housing to heat the internal chamber. In another embodiment of the invention a stream of heated gas may be passed through the chamber in proximity to the liquid receptor, such as through a central cavity of the liquid receptor. In this embodiment the gas stream may be mixed with the evaporated liquid within the internal chamber prior to expulsion of the vapor and gas mixture through the outlet.
In one embodiment of the invention the internal chamber may be pressurized. Preferably the amount of liquid introduced into the internal chamber is accurately measured at ambient pressure and temperature at a flow location upstream from the system for converting. Conventional inexpensive measuring devices, such as a flow meter or metering pump, can be used to measure the amount of liquid supplied to the liquid receptor.
In alternative embodiments of the invention, the system may comprise multiple heated liquid receptors arranged within the housing. The liquid receptors may be arranged within a single internal chamber or multiple chambers may be provided. The invention may further include a splitter for diverting liquid from a primary liquid supply line to each of the heated liquid receptors. Preferably the splitters divert liquid evenly to the circumferential surface of each liquid receptor.
Also described herein is a method for converting a liquid into a vapor. The method includes disposing a, preferably porous, liquid receptor within a housing, delivering the liquid from outside the housing to the liquid receptor via a liquid inlet, and determining the amount of liquid delivered to the liquid receptor with a measuring device. The method also includes heating the liquid delivered to the liquid receptor with a heater to thereby convert the liquid to a vapor, and discharging the vapor outside the housing.
In particular, the method includes the step of providing a liquid receptor for accepting liquid from a liquid source. The liquid receptor is enclosed within a housing. A liquid inlet permits liquid to enter the housing. The method further includes the steps of measuring the amount of liquid delivered from the liquid supply to the liquid inlet, introducing liquid from the liquid inlet into the internal chamber, heating the porous liquid receptor to evaporate all of the liquid introduced into the internal chamber, and discharging the resultant vapor from the internal cavity through the outlet. The invention ensures that all liquid delivered to the liquid receptor is continuously evaporated, even at relatively large liquid flow rates (e.g. up to 500 ml per minute, or more depending upon the required vapor mass flow rate).
In alternative embodiments of the invention, the step of heating the porous liquid receptor may include the step of passing a heated gas through the internal chamber in the vicinity of the liquid receptor. The method may further include the step of mixing the heated gas and the evaporated liquid within the internal chamber of the housing to humidify the gas stream prior to discharging the gas and vapor mixture through the outlet.
In other embodiments of the invention, the method may include the step of pressurizing the internal chamber of the housing. The liquid receptor is heated to a temperature substantially above the boiling temperature of the liquid at the applicable vapor pressure. The step of measuring the volume of fluid preferably is performed at known temperature and pressure, such as ambient temperature and pressure, at a flow location upstream from the liquid inlet.